OS.js is a desktop environment that runs in a browser with fully-fledged window manager, application APIs, GUI toolkit, filesystem abstractions and much more.
Can be installed on all platform and runs in any modern web browser.
This the v3 manual, if you're looking for the legacy v2 manual follow this link.
v3 is scheduled for release this summer/fall. See the TODO list for current goal. This version is fully usable with only some quality of life features missing.